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Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics
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CERAMICS INTERNATIONAL
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Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics Yong Lia, Dawei Wanga, Wenqiang Caob, Bin Lia, Jie Yuanb, Deqing Zhanga,n, Shujun Zhangc,n, Maosheng Caoa,n a
School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, China b School of Information Engineering, MinZu University of China, Beijing 100081, China c Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA Received 2 April 2015; received in revised form 7 April 2015; accepted 7 April 2015
Abstract A novel ferroelectric system, 0.2Pb(Mg1/3Nb2/3)O3–0.8Pb(Sn0.46Ti0.54)O3 with MnO2 addition (PMNST-Mn), was prepared. The dielectric, ferroelectric and piezoelectric properties were investigated. The results demonstrated that the addition of MnO2 suppressed the dielectric relaxor behavior of PMNST. With the increase of MnO2 addition, the diffuse phase transition (DPT) behavior weakened gradually. The addition of MnO2 contributed to the decrease of the dielectric loss (tanδ) and the enhancement of ferroelectric polarization. The optimum ferroelectric and piezoelectric properties were obtained when the addition of MnO2 is 0.75 mol%, and the remnant polarization (Pr) and mechanical quality factor (Qm) were about 25% and 60% higher than those of PMNST, respectively. It was suggested that the formation of oxygen vacancies made the important contribution to suppressing relaxor behavior and improving the electrical properties of PMNST due to the substitution of Mn for B-site. This work provided a practicable strategy to tune electrical properties of ferroelectrics. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Dielectric properties; D. Perovskite; Ferroelectrics; Relaxor
1. Introduction New ferroelectric materials with excellent performance, exhibiting high ferroelectric polarization, giant piezoelectric effect as well as low dielectric loss, continues to attract considerable attention, which motivated by their ferroelectric memory, energy harvesting and piezoelectric applications [1–3]. As a classic relaxor ferroelectric, lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), exhibiting high electrostrictive coefficient, excellent dielectric properties, a broad ferroelectric–paraelectric transition and a frequency dependency of the dielectric properties [4,5], is extensively used in multilayer capacitors, sensors, actuators and dynamic memory applications [6,7]. The relaxor property can be attributed to symmetry breaking composition and structure disorder, which is brought by n
Corresponding authors. E-mail addresses: [email protected] (D. Zhang), [email protected] (S. Zhang), [email protected] (M. Cao).
the differences in valence, ionic radii and electronegativities between Mg2 þ and Nb5 þ ions on the B-site [6]. The applications of single phase PMN are inhibited in various devices due to its difficult preparation and the low transition temperature ( 15 1 C) [8]. Some perovskite materials, such as PbTiO3 (PT), PbHfO3 (PH), Pb(Zr,Ti)O3 (PZT), Pb(Zn1/3Nb2/3)O3 (PZN) or Pb(Sc1/2Nb1/ 2)O3 (PSN), are added into PMN to form ferroelectrics polysystem to solve those problems [9–13]. Particularly, Pb(SnxTi1 x)O3 (PST), showing a behavior analogous to PZT [14], is mixed with PMN to create a new ferroelectric, Pb(Mg1/3Nb2/3)O3–Pb (SnxTi1 x)O3 (PMNST), which possesses high transition temperature, good piezoelectric and ferroelectric properties [15]. Unfortunately, the studies on improving the electrical properties of PMNST have been carried out rarely, which are extremely crucial for the development of PMNST in practical applications. Manganese is one of the most effective acceptor dopants. It was reported that Mn doping generated oxygen vacancy around Mn2 þ , and vacancy and ion interacted to form a charge–dipole
http://dx.doi.org/10.1016/j.ceramint.2015.04.030 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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[16]. The addition of Mn lowered sintering temperature without sacrificing the piezoelectric performance [17]. Mn doping also had obvious influence on the leakage, relaxor, aging behavior and strain properties of ferroelectrics [18–21]. Moreover, Mn ions reduced dispersion in dielectric constant [22], and improved the mechanical quality factor as well as piezoelectric properties [23– 25]. In this work, MnO2 added PMNST ceramics were synthesized. The dielectric, ferroelectric and piezoelectric properties of the ferroelectrics were investigated systematically. 2. Experimental The compositions PMNST and PMNST-Mn with MnO2 molar ratios of 0.50, 0.75 and 1.00 mol% ceramics (abbreviated as PMNST-0.50Mn, PMNST-0.75Mn, PMNST1.00Mn) were perpared by a conventional solid state method. Fristly, MgNb2O6 was synthesized by using the powders of 3MgCO3 Mg(OH)2 3H2O (99%) and Nb2O5 (99%). Stoichiometric amount of PbO (99%), SnO2 (99%), TiO2 (99%), MgNb2O6 and MnO2 (99%) were wet-milled in alcohol for 24 h, and an excess of 2 mol% PbO was added. Then the powders were calcined at 800 1C for 4 h, and subsequently milled in alcohol for 12 h. The dried powders were grinded in a mortar and then dry-pressed into pellets (12 mm in diameter) with 6 wt% polyvinyl alcohol (PVA) liquid binder. The pellets of PMNST-xMn were sintered at 1130 1C for 2 h in a close crucible containing a PbZrO3 lead source. Finally, they were treated through an annealing process at 900 1C for 5 h. The phase and morphologies of the samples were measured using X-ray powder diffraction (XRD, Ni-filtered Cu Kα radiation, 40 kV) and scanning electron microscopy (SEM; Hitachi S-4800). Silver paste was printed as electrodes on the both sides of the disk samples and then fired at 700 1C for 10 min. Poling was carried out in silicone oil at 120 1C for 10 min with an electric field of 30 kV/cm. The dielectric property was measured using a multi-frequency inductance capacitance resistance (LCR) analyzer (Agilent E4980A, Santa Clara, CA) with an automated temperature controller. The polarization–electric field hysteresis loops were measured using a Radiant Precision Workstation ferroelectric tester system (Radiant Technology, Albuquerque, NM). Piezoelectric coefficients were measured on the disk samples using a Berlincourt d33 meter (ZJ-2; Institute of Acoustics Academia Sinica, Beijing, China). The planar electromechanical coupling kp and mechanical quality factor Qm were determined from the resonance and anti resonance frequencies, which were measured using an Impedance/Gain-phase analyzer (HP 4194 A). The Archimedes method was used to measure the density of the ceramics. 3. Results and discussion 3.1. Structure analysis The XRD patterns of the PMNST and PMNST-Mn ceramics specimens are shown in Fig. 1. All the specimens, which are of tetragonal symmetry at room temperature, possess perovskite
Fig. 1. XRD patterns of the PMNST and PMNST-Mn ceramics with different MnO2 additions.
structure without detectable traces of pyrochlore or other impurities. With the increase of MnO2 addition, the lattice constant a decreases from 4.0378 Å to 4.0285 Å, and c remains in about 3.9950 Å. The decrease of unit cell size is related to oxygen vacancies, which are formed to compensate the charge due to the aliovalent substitution of Mn ions for B-site ions. Fig. 2 shows the SEM photographs of the PMNST and PMNST-Mn ceramics. The densification and grain size of the ceramics are obviously changed with the increase of MnO2 addition. The PMNST-Mn has higher density compared with the PMNST which exhibits loose microstructure morphology with pores. For the PMNST-0.50Mn and PMNST-0.75Mn, the grain size increases (Fig. 2b,c), due to the high sintering activity via the partial substitution for B-site [26]. However, for the PMNST1.00Mn, the grain size decreases remarkably (Fig. 2d). The insets show the distribution of grain size for the PMNST and PMNSTMn ceramics, where the number of grains is more than 300 in the calculation of grain size distribution. The grain size of the PMNST distributes uniformly between 4 and 10 μm, while the grain size of the PMNST-0.50Mn and PMNST-0.75Mn concentrates around 7–10 μm. The PMNST-1.00Mn exhibits the decreasing grain size, which is on the order of 4–8 μm and concentrates around 5–6 μm. These results indicate that suitable MnO2 addition facilitates the growth of grain as well as the densification of the ceramics. 3.2. Dielectric properties The temperature dependence of dielectric constant (εr) and dielectric loss (tanδ) of the PMNST and PMNST-Mn at 100, 1 k, 10 k and 100 kHz is shown in Fig. 3. The tanδ decreases with the increase of MnO2 addition, and the minimum tanδ is obtained in the PMNST-0.75Mn. The pinning effect of oxygen vacancies on domain walls is responsible for the decrease of the tanδ. At a selected temperature and frequency, the εr of the PMNST is little higher than those of PMNST-Mn. For the PMNST, the εr shows an obvious frequency dispersion (Fig. 3a). With the increase of MnO2 addition, the frequency dispersion of the εr for the PMNST-Mn is weakened, indicating that relaxor behavior is suppressed.
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Fig. 2. SEM micrographs and distribution of grain size for the PMNST (a) and PMNST-Mn ceramics with different MnO2 additions: 0.50 mol% (b), 0.75 mol% (c), 1.00 mol% (d).
For a ferroelectric, the dielectric constant above the Curie temperature follows the Curie–Weiss law ε¼
C ðT 4 T C Þ; T TC
ð1Þ
where C is the Curie–Weiss constant, and TC is the Curie– Weiss temperature. Fig. 4 shows the inverse dielectric constant (10,000/εr) as a function of temperature for the PMNST and PMNST-Mn at 100 kHz. The plots agree with the Curie–Weiss law as there is a close correlation between the experimental data and the fitting line by Eq. (1) above TC. The deviation (ΔTm) from the Curie– Weiss law can be expressed as [27] ΔT m ¼ T cw –T m ;
ð2Þ
where Tm is the temperature in which the dielectric constant reaches its maximum, Tcw represents the temperature from which the dielectric constant starts to deviate from the Curie–Weiss law. The values of the Tm, Tcw and ΔTm are listed in Table 1. With the increase of MnO2 addition, the Tcw decreases while the Tm slightly increases. The value of ΔTm decreases from
75 1C to 46 1C. These results indicate that the diffuse phase transition (DPT) behavior of PMNST-Mn is weakened gradually with the increase of MnO2 addition. For the relaxor ferroelectrics, the reciprocal of the dielectric constant and the temperature obey the Uchino and Nomura function [28] 1 1 ðT T m Þγ ; ¼ εr εm C
ð3Þ
where εm is the maximum value of the dielectric constant, γ is the indicator of the degree of diffuseness. Fig. 5 shows the plots of ln(1/εr 1/εm) versus ln(T Tm) for the PMNST and PMNST-Mn. It is observed that the plots obey a linear relationship. The slope of the fitting curves is used to determine the value of γ. With the increase of MnO2 addition, the value of γ decreases from 1.92 to 1.73. It is known that the limiting values γ¼ 1 and γ¼ 2 are the character for normal ferroelectric phase transition and for complete diffuse phase transition, respectively [29]. Hence, this result demonstrates that the addition of MnO2 weakens the DPT behavior of PMNST. We suggest that the formation of oxygen vacancies is accountable for the decrease
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Fig. 3. The temperature dependence of dielectric constant (εr) of the PMNST and PMNST-Mn ceramics at 100, 1 k, 10 k and 100 kHz.
Fig. 4. The inverse dielectric constant (10,000/εr) as a function of temperature for the PMNST and PMNST-Mn ceramics at 100 kHz. Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
Y. Li et al. / Ceramics International ] (]]]]) ]]]–]]] Table 1 The temperature of the maximum dielectric constant Tm, temperature of the dielectric constant that follows the Curie–Weiss law Tcw, deviation ΔTm, and the degree of diffuseness γ for the PMNST and PMNST-Mn at 100 kHz. Compositions
Tm (1C)
Tcw (1C)
ΔTm (1C)
γ
PMNST PMNST-0.50Mn PMNST-0.75Mn PMNST-1.00Mn
179 183 185 189
254 244 238 235
75 61 53 46
1.96 1.88 1.79 1.73
5
Fig. 7 shows the ferroelectric properties as a function of electric fields. With the increase of electric fields, the remnant polarization (Pr) and coercive field (EC) increase. At the same electric field, the Pr and EC of the PMNST-0.75Mn are larger than those of the PMNST. The polarization hysteresis electric field loops are drawn for the PMNST and PMNST-0.75Mn before and after poling, as shown in Fig. 8. After poling, the Pr increases 237% and 269% for the PMNST and PMNST-0.75Mn, respectively, and the coercive field (EC) increases as well. Compared with the poled PMNST, the poled PMNST-0.75Mn have higher Pr, where the Pr increases from 20.55 to 25.10 μC/cm2. The defect structure on the domains configuration and the interaction of defects with domain walls show extremely important function [34,35]. It can be deduced that remnant polarization is related with the evolution of domains structure and the function of defect structure. During the poling process, external electric field promotes domains reorientation and growth for the PMNST and PMNST-0.75Mn, leading to the increase of the Pr. Moreover, for the PMNST-Mn, the defect dipoles between the acceptor ions and charge compensating oxygen vacancies also have an important contribution to polarization. The defect dipoles can be reorientated after applying an external field, resulting in improving ferroelectric properties [36].
Fig. 5. The plots of ln(1/εr 1/εm) as a function of ln(T Tm) for the PMNST and PMNST-Mn ceramics.
of DPT degree. The formation of oxygen vacancies promotes rearrangement of B-site ions to enhance the degree of order in B-site ions [30,31]. In addition, the oxygen vacancies as pinning center prevent domain walls motion, leading to weakening the relaxor behavior as well [32]. 3.3. Ferroelectric properties Fig. 6 shows the polarization hysteresis electric field loops for the unpoled PMNST and PMNST-0.75Mn at varying electric fields. The PMNST has slim hysteresis loop, revealing a characteristic of relaxor ferroelectrics [33]. The PMNST0.75Mn exhibits square-like hysteresis loop, a normal ferroelectric characteristic. This also indicates that the PMNST change from relaxor to normal ferroelectrics as MnO2 is added.
Fig. 6. The polarization hysteresis electric field loops of the PMNST and PMNST-0.75Mn ceramics at varying electric fields.
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Fig. 7. The ferroelectric properties of the PMNST and PMNST-0.75Mn ceramics as a function of electric fields.
The ferroelectric properties of the poled PMNST and PMNSTMn ceramics are listed in Table 2. With the increase of MnO2 addition, the Pr increases at first and then decreases, indicating that excess addition has a negative influence on ferroelectric properties of PMNST-Mn ceramics. 3.4. Piezoelectric properties Fig. 9 shows the piezoelectric coefficient (d33), planar electromechanical coupling (kp), mechanical quality and factor (Qm) as the function of the addition of MnO2. It is found that the Qm increases firstly and then decreases. The Qm reaches the maximum value 554 in the PMNST-0.75Mn, which is about 60% higher than that of the PMNST. The oxygen vacancies act as pinning points to prevent domain walls from moving, thus the addition of MnO2 has a hardening effect on PMNST. The addition of MnO2 does not deteriorate other piezoelectric properties. The most excellent piezoelectric properties are obtained in the PMNST-0.75Mn, where the kp reach the maximum value 57.0%, and the d33 is maintained at 430 pC/N. 4. Conclusions In conclusion, MnO2 added PMNST ceramics were perpared by a conventional solid state method. With the increase of MnO2, the densification of PMNST was improved, and the grain size of the ceramics increased at first and then decreased. PMNST exhibited weakened relaxor behavior with the increase of MnO2
Fig. 8. The polarization hysteresis electric field loops for the PMNST and PMNST-0.75Mn ceramics before and after poling.
Table 2 The densities and electric properties of the PMNST and PMNST-Mn at room temperature (density ρ, relative density ρr, coercive field EC, remnant polarization Pr, mechanical quality factor Qm, piezoelectric coefficient d33, and planar electromechanical coupling kp). ρr Compositions ρ (g/cm3) (%)
EC Pr (kV/cm) (μC/cm2)
Qm d33 (pC/N)
kp
PMNST PMNST0.50Mn PMNST0.75Mn PMNST1.00Mn
8.22 8.32
95.4 96.7
7.62 7.91
20.55 24.90
360 430 497 400
0.52 0.53
8.45
98.3
8.13
25.10
554 430
0.57
8.36
97.2
7.90
23.06
420 420
0.56
addition. This was related to the formation of oxygen vacancies, which enhanced the degree of order in B-site ions. MnO2 addition also reduced the dielectric loss and improved the ferroelectric and piezoelectric properties of PMNST. The most excellent electrical properties were obtained in the PMNST-0.75Mn. This work provides a novel strategy to tune dielectric relaxor and to optimize ferroelectric and piezoelectric properties of ferroelectrics. It
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Fig. 9. The piezoelectric coefficient (d33), planar electromechanical coupling (kp), and mechanical quality factor (Qm) as the function of MnO2 additions.
demonstrates potential application of ferroelectrics materials in memory, energy harvesting and high-power piezoelectric fields. Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 51132002, 51372282, 51402005 and 51272110). References [1] X.P. Wang, J.G. Wu, D.Q. Xiao, J.G. Zhu, X.J. Cheng, et al., Giant piezoelectricity in potassium–sodium niobate lead-free ceramics, J. Am. Chem. Soc. 136 (2014) 2905–2910. [2] J.G. Wu, D.Q. Xiao, J.G. Zhu, Potassium–sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries, Chem. Rev. 115 (2015) 2559–2595, http://dx.doi.org/10.1021/cr5006809. [3] A.A. Bokov, Z.G. Ye, Recent progress in relaxor ferroelectrics with perovskite structure, J. Mater. Sci. 41 (2006) 31–52. [4] S.W. Choi, T.R. Shrout, S.J. Jang, A.S. Bhalla, Dielectric and pyroelectric properties in the Pb(Mg1/3Nb2/3)O3–PbTiO3 system, Ferroelectrics 100 (1989) 29–38. [5] S.L. Swartz, T.R. Shrout, W.A. Schulze, L.E. Cross, Dielectric properties of lead–magnesium niobate ceramics, J. Am. Ceram. Soc. 67 (1984) 311–315. [6] Y.H. Chen, K. Uchino, M. Shen, D. Viehland, Substituent effects on the mechanical quality factor of Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Sc1/2Nb1/2) O3–PbTiO3 ceramics, J. Appl. Phys. 90 (2001) 1455. [7] K. Uchino, Electrostrictive actuators, materials and applications, Am. Ceram. Soc. Bull. 65 (1986) 647–652. [8] C.H. Zhang, Z. Xu, J.J. Gao, X. Yao, Study on the dielectric properties of 0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 ceramic under hydrostatic pressure, Ferroelectrics 401 (2010) 218–225. [9] A.L. Costa, C. Galassi, G. Fabri, E. Roncari, C. Capiani, Pyrochlore phase and microstructure development in lead magnesium niobate materials, J. Eur. Ceram. Soc. 21 (2001) 1165–1170.
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Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics Yong Lia, Dawei Wanga, Wenqiang Caob, Bin Lia, Jie Yuanb, Deqing Zhanga,n, Shujun Zhangc,n, Maosheng Caoa,n a
School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, China b School of Information Engineering, MinZu University of China, Beijing 100081, China c Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA Received 2 April 2015; received in revised form 7 April 2015; accepted 7 April 2015
Abstract A novel ferroelectric system, 0.2Pb(Mg1/3Nb2/3)O3–0.8Pb(Sn0.46Ti0.54)O3 with MnO2 addition (PMNST-Mn), was prepared. The dielectric, ferroelectric and piezoelectric properties were investigated. The results demonstrated that the addition of MnO2 suppressed the dielectric relaxor behavior of PMNST. With the increase of MnO2 addition, the diffuse phase transition (DPT) behavior weakened gradually. The addition of MnO2 contributed to the decrease of the dielectric loss (tanδ) and the enhancement of ferroelectric polarization. The optimum ferroelectric and piezoelectric properties were obtained when the addition of MnO2 is 0.75 mol%, and the remnant polarization (Pr) and mechanical quality factor (Qm) were about 25% and 60% higher than those of PMNST, respectively. It was suggested that the formation of oxygen vacancies made the important contribution to suppressing relaxor behavior and improving the electrical properties of PMNST due to the substitution of Mn for B-site. This work provided a practicable strategy to tune electrical properties of ferroelectrics. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Dielectric properties; D. Perovskite; Ferroelectrics; Relaxor
1. Introduction New ferroelectric materials with excellent performance, exhibiting high ferroelectric polarization, giant piezoelectric effect as well as low dielectric loss, continues to attract considerable attention, which motivated by their ferroelectric memory, energy harvesting and piezoelectric applications [1–3]. As a classic relaxor ferroelectric, lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), exhibiting high electrostrictive coefficient, excellent dielectric properties, a broad ferroelectric–paraelectric transition and a frequency dependency of the dielectric properties [4,5], is extensively used in multilayer capacitors, sensors, actuators and dynamic memory applications [6,7]. The relaxor property can be attributed to symmetry breaking composition and structure disorder, which is brought by n
Corresponding authors. E-mail addresses: [email protected] (D. Zhang), [email protected] (S. Zhang), [email protected] (M. Cao).
the differences in valence, ionic radii and electronegativities between Mg2 þ and Nb5 þ ions on the B-site [6]. The applications of single phase PMN are inhibited in various devices due to its difficult preparation and the low transition temperature ( 15 1 C) [8]. Some perovskite materials, such as PbTiO3 (PT), PbHfO3 (PH), Pb(Zr,Ti)O3 (PZT), Pb(Zn1/3Nb2/3)O3 (PZN) or Pb(Sc1/2Nb1/ 2)O3 (PSN), are added into PMN to form ferroelectrics polysystem to solve those problems [9–13]. Particularly, Pb(SnxTi1 x)O3 (PST), showing a behavior analogous to PZT [14], is mixed with PMN to create a new ferroelectric, Pb(Mg1/3Nb2/3)O3–Pb (SnxTi1 x)O3 (PMNST), which possesses high transition temperature, good piezoelectric and ferroelectric properties [15]. Unfortunately, the studies on improving the electrical properties of PMNST have been carried out rarely, which are extremely crucial for the development of PMNST in practical applications. Manganese is one of the most effective acceptor dopants. It was reported that Mn doping generated oxygen vacancy around Mn2 þ , and vacancy and ion interacted to form a charge–dipole
http://dx.doi.org/10.1016/j.ceramint.2015.04.030 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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[16]. The addition of Mn lowered sintering temperature without sacrificing the piezoelectric performance [17]. Mn doping also had obvious influence on the leakage, relaxor, aging behavior and strain properties of ferroelectrics [18–21]. Moreover, Mn ions reduced dispersion in dielectric constant [22], and improved the mechanical quality factor as well as piezoelectric properties [23– 25]. In this work, MnO2 added PMNST ceramics were synthesized. The dielectric, ferroelectric and piezoelectric properties of the ferroelectrics were investigated systematically. 2. Experimental The compositions PMNST and PMNST-Mn with MnO2 molar ratios of 0.50, 0.75 and 1.00 mol% ceramics (abbreviated as PMNST-0.50Mn, PMNST-0.75Mn, PMNST1.00Mn) were perpared by a conventional solid state method. Fristly, MgNb2O6 was synthesized by using the powders of 3MgCO3 Mg(OH)2 3H2O (99%) and Nb2O5 (99%). Stoichiometric amount of PbO (99%), SnO2 (99%), TiO2 (99%), MgNb2O6 and MnO2 (99%) were wet-milled in alcohol for 24 h, and an excess of 2 mol% PbO was added. Then the powders were calcined at 800 1C for 4 h, and subsequently milled in alcohol for 12 h. The dried powders were grinded in a mortar and then dry-pressed into pellets (12 mm in diameter) with 6 wt% polyvinyl alcohol (PVA) liquid binder. The pellets of PMNST-xMn were sintered at 1130 1C for 2 h in a close crucible containing a PbZrO3 lead source. Finally, they were treated through an annealing process at 900 1C for 5 h. The phase and morphologies of the samples were measured using X-ray powder diffraction (XRD, Ni-filtered Cu Kα radiation, 40 kV) and scanning electron microscopy (SEM; Hitachi S-4800). Silver paste was printed as electrodes on the both sides of the disk samples and then fired at 700 1C for 10 min. Poling was carried out in silicone oil at 120 1C for 10 min with an electric field of 30 kV/cm. The dielectric property was measured using a multi-frequency inductance capacitance resistance (LCR) analyzer (Agilent E4980A, Santa Clara, CA) with an automated temperature controller. The polarization–electric field hysteresis loops were measured using a Radiant Precision Workstation ferroelectric tester system (Radiant Technology, Albuquerque, NM). Piezoelectric coefficients were measured on the disk samples using a Berlincourt d33 meter (ZJ-2; Institute of Acoustics Academia Sinica, Beijing, China). The planar electromechanical coupling kp and mechanical quality factor Qm were determined from the resonance and anti resonance frequencies, which were measured using an Impedance/Gain-phase analyzer (HP 4194 A). The Archimedes method was used to measure the density of the ceramics. 3. Results and discussion 3.1. Structure analysis The XRD patterns of the PMNST and PMNST-Mn ceramics specimens are shown in Fig. 1. All the specimens, which are of tetragonal symmetry at room temperature, possess perovskite
Fig. 1. XRD patterns of the PMNST and PMNST-Mn ceramics with different MnO2 additions.
structure without detectable traces of pyrochlore or other impurities. With the increase of MnO2 addition, the lattice constant a decreases from 4.0378 Å to 4.0285 Å, and c remains in about 3.9950 Å. The decrease of unit cell size is related to oxygen vacancies, which are formed to compensate the charge due to the aliovalent substitution of Mn ions for B-site ions. Fig. 2 shows the SEM photographs of the PMNST and PMNST-Mn ceramics. The densification and grain size of the ceramics are obviously changed with the increase of MnO2 addition. The PMNST-Mn has higher density compared with the PMNST which exhibits loose microstructure morphology with pores. For the PMNST-0.50Mn and PMNST-0.75Mn, the grain size increases (Fig. 2b,c), due to the high sintering activity via the partial substitution for B-site [26]. However, for the PMNST1.00Mn, the grain size decreases remarkably (Fig. 2d). The insets show the distribution of grain size for the PMNST and PMNSTMn ceramics, where the number of grains is more than 300 in the calculation of grain size distribution. The grain size of the PMNST distributes uniformly between 4 and 10 μm, while the grain size of the PMNST-0.50Mn and PMNST-0.75Mn concentrates around 7–10 μm. The PMNST-1.00Mn exhibits the decreasing grain size, which is on the order of 4–8 μm and concentrates around 5–6 μm. These results indicate that suitable MnO2 addition facilitates the growth of grain as well as the densification of the ceramics. 3.2. Dielectric properties The temperature dependence of dielectric constant (εr) and dielectric loss (tanδ) of the PMNST and PMNST-Mn at 100, 1 k, 10 k and 100 kHz is shown in Fig. 3. The tanδ decreases with the increase of MnO2 addition, and the minimum tanδ is obtained in the PMNST-0.75Mn. The pinning effect of oxygen vacancies on domain walls is responsible for the decrease of the tanδ. At a selected temperature and frequency, the εr of the PMNST is little higher than those of PMNST-Mn. For the PMNST, the εr shows an obvious frequency dispersion (Fig. 3a). With the increase of MnO2 addition, the frequency dispersion of the εr for the PMNST-Mn is weakened, indicating that relaxor behavior is suppressed.
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Fig. 2. SEM micrographs and distribution of grain size for the PMNST (a) and PMNST-Mn ceramics with different MnO2 additions: 0.50 mol% (b), 0.75 mol% (c), 1.00 mol% (d).
For a ferroelectric, the dielectric constant above the Curie temperature follows the Curie–Weiss law ε¼
C ðT 4 T C Þ; T TC
ð1Þ
where C is the Curie–Weiss constant, and TC is the Curie– Weiss temperature. Fig. 4 shows the inverse dielectric constant (10,000/εr) as a function of temperature for the PMNST and PMNST-Mn at 100 kHz. The plots agree with the Curie–Weiss law as there is a close correlation between the experimental data and the fitting line by Eq. (1) above TC. The deviation (ΔTm) from the Curie– Weiss law can be expressed as [27] ΔT m ¼ T cw –T m ;
ð2Þ
where Tm is the temperature in which the dielectric constant reaches its maximum, Tcw represents the temperature from which the dielectric constant starts to deviate from the Curie–Weiss law. The values of the Tm, Tcw and ΔTm are listed in Table 1. With the increase of MnO2 addition, the Tcw decreases while the Tm slightly increases. The value of ΔTm decreases from
75 1C to 46 1C. These results indicate that the diffuse phase transition (DPT) behavior of PMNST-Mn is weakened gradually with the increase of MnO2 addition. For the relaxor ferroelectrics, the reciprocal of the dielectric constant and the temperature obey the Uchino and Nomura function [28] 1 1 ðT T m Þγ ; ¼ εr εm C
ð3Þ
where εm is the maximum value of the dielectric constant, γ is the indicator of the degree of diffuseness. Fig. 5 shows the plots of ln(1/εr 1/εm) versus ln(T Tm) for the PMNST and PMNST-Mn. It is observed that the plots obey a linear relationship. The slope of the fitting curves is used to determine the value of γ. With the increase of MnO2 addition, the value of γ decreases from 1.92 to 1.73. It is known that the limiting values γ¼ 1 and γ¼ 2 are the character for normal ferroelectric phase transition and for complete diffuse phase transition, respectively [29]. Hence, this result demonstrates that the addition of MnO2 weakens the DPT behavior of PMNST. We suggest that the formation of oxygen vacancies is accountable for the decrease
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Fig. 3. The temperature dependence of dielectric constant (εr) of the PMNST and PMNST-Mn ceramics at 100, 1 k, 10 k and 100 kHz.
Fig. 4. The inverse dielectric constant (10,000/εr) as a function of temperature for the PMNST and PMNST-Mn ceramics at 100 kHz. Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
Y. Li et al. / Ceramics International ] (]]]]) ]]]–]]] Table 1 The temperature of the maximum dielectric constant Tm, temperature of the dielectric constant that follows the Curie–Weiss law Tcw, deviation ΔTm, and the degree of diffuseness γ for the PMNST and PMNST-Mn at 100 kHz. Compositions
Tm (1C)
Tcw (1C)
ΔTm (1C)
γ
PMNST PMNST-0.50Mn PMNST-0.75Mn PMNST-1.00Mn
179 183 185 189
254 244 238 235
75 61 53 46
1.96 1.88 1.79 1.73
5
Fig. 7 shows the ferroelectric properties as a function of electric fields. With the increase of electric fields, the remnant polarization (Pr) and coercive field (EC) increase. At the same electric field, the Pr and EC of the PMNST-0.75Mn are larger than those of the PMNST. The polarization hysteresis electric field loops are drawn for the PMNST and PMNST-0.75Mn before and after poling, as shown in Fig. 8. After poling, the Pr increases 237% and 269% for the PMNST and PMNST-0.75Mn, respectively, and the coercive field (EC) increases as well. Compared with the poled PMNST, the poled PMNST-0.75Mn have higher Pr, where the Pr increases from 20.55 to 25.10 μC/cm2. The defect structure on the domains configuration and the interaction of defects with domain walls show extremely important function [34,35]. It can be deduced that remnant polarization is related with the evolution of domains structure and the function of defect structure. During the poling process, external electric field promotes domains reorientation and growth for the PMNST and PMNST-0.75Mn, leading to the increase of the Pr. Moreover, for the PMNST-Mn, the defect dipoles between the acceptor ions and charge compensating oxygen vacancies also have an important contribution to polarization. The defect dipoles can be reorientated after applying an external field, resulting in improving ferroelectric properties [36].
Fig. 5. The plots of ln(1/εr 1/εm) as a function of ln(T Tm) for the PMNST and PMNST-Mn ceramics.
of DPT degree. The formation of oxygen vacancies promotes rearrangement of B-site ions to enhance the degree of order in B-site ions [30,31]. In addition, the oxygen vacancies as pinning center prevent domain walls motion, leading to weakening the relaxor behavior as well [32]. 3.3. Ferroelectric properties Fig. 6 shows the polarization hysteresis electric field loops for the unpoled PMNST and PMNST-0.75Mn at varying electric fields. The PMNST has slim hysteresis loop, revealing a characteristic of relaxor ferroelectrics [33]. The PMNST0.75Mn exhibits square-like hysteresis loop, a normal ferroelectric characteristic. This also indicates that the PMNST change from relaxor to normal ferroelectrics as MnO2 is added.
Fig. 6. The polarization hysteresis electric field loops of the PMNST and PMNST-0.75Mn ceramics at varying electric fields.
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Fig. 7. The ferroelectric properties of the PMNST and PMNST-0.75Mn ceramics as a function of electric fields.
The ferroelectric properties of the poled PMNST and PMNSTMn ceramics are listed in Table 2. With the increase of MnO2 addition, the Pr increases at first and then decreases, indicating that excess addition has a negative influence on ferroelectric properties of PMNST-Mn ceramics. 3.4. Piezoelectric properties Fig. 9 shows the piezoelectric coefficient (d33), planar electromechanical coupling (kp), mechanical quality and factor (Qm) as the function of the addition of MnO2. It is found that the Qm increases firstly and then decreases. The Qm reaches the maximum value 554 in the PMNST-0.75Mn, which is about 60% higher than that of the PMNST. The oxygen vacancies act as pinning points to prevent domain walls from moving, thus the addition of MnO2 has a hardening effect on PMNST. The addition of MnO2 does not deteriorate other piezoelectric properties. The most excellent piezoelectric properties are obtained in the PMNST-0.75Mn, where the kp reach the maximum value 57.0%, and the d33 is maintained at 430 pC/N. 4. Conclusions In conclusion, MnO2 added PMNST ceramics were perpared by a conventional solid state method. With the increase of MnO2, the densification of PMNST was improved, and the grain size of the ceramics increased at first and then decreased. PMNST exhibited weakened relaxor behavior with the increase of MnO2
Fig. 8. The polarization hysteresis electric field loops for the PMNST and PMNST-0.75Mn ceramics before and after poling.
Table 2 The densities and electric properties of the PMNST and PMNST-Mn at room temperature (density ρ, relative density ρr, coercive field EC, remnant polarization Pr, mechanical quality factor Qm, piezoelectric coefficient d33, and planar electromechanical coupling kp). ρr Compositions ρ (g/cm3) (%)
EC Pr (kV/cm) (μC/cm2)
Qm d33 (pC/N)
kp
PMNST PMNST0.50Mn PMNST0.75Mn PMNST1.00Mn
8.22 8.32
95.4 96.7
7.62 7.91
20.55 24.90
360 430 497 400
0.52 0.53
8.45
98.3
8.13
25.10
554 430
0.57
8.36
97.2
7.90
23.06
420 420
0.56
addition. This was related to the formation of oxygen vacancies, which enhanced the degree of order in B-site ions. MnO2 addition also reduced the dielectric loss and improved the ferroelectric and piezoelectric properties of PMNST. The most excellent electrical properties were obtained in the PMNST-0.75Mn. This work provides a novel strategy to tune dielectric relaxor and to optimize ferroelectric and piezoelectric properties of ferroelectrics. It
Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030
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Fig. 9. The piezoelectric coefficient (d33), planar electromechanical coupling (kp), and mechanical quality factor (Qm) as the function of MnO2 additions.
demonstrates potential application of ferroelectrics materials in memory, energy harvesting and high-power piezoelectric fields. Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 51132002, 51372282, 51402005 and 51272110). References [1] X.P. Wang, J.G. Wu, D.Q. Xiao, J.G. Zhu, X.J. Cheng, et al., Giant piezoelectricity in potassium–sodium niobate lead-free ceramics, J. Am. Chem. Soc. 136 (2014) 2905–2910. [2] J.G. Wu, D.Q. Xiao, J.G. Zhu, Potassium–sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries, Chem. Rev. 115 (2015) 2559–2595, http://dx.doi.org/10.1021/cr5006809. [3] A.A. Bokov, Z.G. Ye, Recent progress in relaxor ferroelectrics with perovskite structure, J. Mater. Sci. 41 (2006) 31–52. [4] S.W. Choi, T.R. Shrout, S.J. Jang, A.S. Bhalla, Dielectric and pyroelectric properties in the Pb(Mg1/3Nb2/3)O3–PbTiO3 system, Ferroelectrics 100 (1989) 29–38. [5] S.L. Swartz, T.R. Shrout, W.A. Schulze, L.E. Cross, Dielectric properties of lead–magnesium niobate ceramics, J. Am. Ceram. Soc. 67 (1984) 311–315. [6] Y.H. Chen, K. Uchino, M. Shen, D. Viehland, Substituent effects on the mechanical quality factor of Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Sc1/2Nb1/2) O3–PbTiO3 ceramics, J. Appl. Phys. 90 (2001) 1455. [7] K. Uchino, Electrostrictive actuators, materials and applications, Am. Ceram. Soc. Bull. 65 (1986) 647–652. [8] C.H. Zhang, Z. Xu, J.J. Gao, X. Yao, Study on the dielectric properties of 0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 ceramic under hydrostatic pressure, Ferroelectrics 401 (2010) 218–225. [9] A.L. Costa, C. Galassi, G. Fabri, E. Roncari, C. Capiani, Pyrochlore phase and microstructure development in lead magnesium niobate materials, J. Eur. Ceram. Soc. 21 (2001) 1165–1170.
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Please cite this article as: Y. Li, et al., Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.030